Lean Body Mass Calculator

Lean body mass is everything that is not fat: muscle, bone, organs, water and connective tissue. It is what you want to keep while dieting.

Why lean body mass matters

Lean body mass — sometimes called fat-free mass — is the sum of muscle, bone, organs, skin, connective tissue and body water. It is the metabolically expensive part of you, and it drives several things at once:

• Resting metabolism. The Katch-McArdle equation predicts BMR from lean mass alone, and for lean or muscular people it beats the weight-based equations.
• Diet quality. Losing weight is trivial; losing fat while keeping lean mass is the actual goal. Tracking lean mass tells you which one you are doing.
• Protein needs. Requirements scale with lean mass rather than total weight, which matters a great deal at higher body fat levels.
• Drug dosing and function. Clinically, lean mass predicts the volume of distribution of many drugs better than total weight does.

The three estimation formulas

Boer is generally regarded as the best all-round performer and is the one most often used clinically. James is known to behave badly at high body weights, where the squared term can drive the estimate down implausibly. Hume sits between them.

If you supply a body fat percentage, none of this guesswork is needed: lean mass is simply weight × (1 − body fat ÷ 100), and that is what the calculator reports instead.

Keeping lean mass during a diet

In an uncontrolled calorie deficit, something like 25 % of the weight lost comes from lean tissue. Three interventions cut that substantially, and they are well evidenced:

1. Resistance training, two to four times a week. This is the strongest signal telling the body that muscle is worth keeping. Nothing else substitutes for it.
2. Adequate protein. Roughly 1.6–2.2 g per kilogram of lean mass per day during a deficit. Our protein calculator works it out.
3. A moderate deficit. Around 500 kcal a day rather than 1,000. Aggressive deficits reliably cost more lean tissue for each kilogram lost.

Sleep matters too: short sleep during a deficit shifts the composition of the loss towards lean tissue in controlled trials.